Emergency lighting system

ABSTRACT

An emergency lighting system for electric discharge lamps utilizing an AC - DC inverter to supply emergency power and having a normally non-conductive switching element in the feedback circuit of the inverter which enables the inverter upon failure of line power.

United States Patent Chandler 1 May 2,1972

[54] EMERGENCY LIGHTING SYSTEM [72] Inventor: Edward A. Chandler, Bois des Filion,

Quebec, Canada [73] Assignee: Tenelux Limited, Bois des Filion, Province of Quebec, Canada [22] Filed: Apr. 6, 1970 [21] App]. No.: 25,879

[30] Foreign'Application Priority Data Apr. 8, 1969 Canada ..048,l43

[52] US. Cl ..3I5/86,315/l2 7,315/l71 I 315/200,3l5/202 [51] Int. Cl. v. ..H05b 41/14 [58] Field of Search ..307/64, 66; 240/37.1, 51.11; 315/86, 87, 88, 127, 171, 200, 202, 260

[5 6] References Cited UNITED STATES PATENTS 3,356,891 12/1967 Godard ..7.. V.I 51$"/86 3,435,206 3/1969 Swanson ..240/51. 1 1 X 3,283,144 11/1966 ....240/51.11 3,311,744 3/1967 ....240/51.ll 3,255,358 6/1966 Kilpatrick ..307/64 FOREIGN PATENTS OR APPLlCATlONS 1,079,967 8/1967 Great Britain ..315/86 Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter Attorney-Alan Swabey ABSTRACT An emergency lighting system for electric discharge lamps utilizing an AC DC inverter to supply emergency power and having a normally non-conductive switching element in the feedback circuit of the inverter which enables the inverter upon failure of line power.

9 Claims, 2 Drawing Figures Patented May 2, 1972 3,660,714

2 Sheets-Sheet l F/ 9W1 INVENTOR 1. Edward A. CHANDLER ATTORNEY Patented May 2, 1972 lwl IL 2 l 2 l m I A TTORNEY EMERGENCY LIGHTING SYSTEM This invention relates to improvements in electric lighting systems and more particularly relates to emergency lighting systems which utilize fluorescent or similar type electric discharge lamps.

At the present time, to provide emergency lighting automatically when a power failure occurs, it is a common practice to use separate emergency lighting circuits which may be selfcontained systems comprising one or more lamps and a battery together with a battery charging circuit and a relay or other type of switching device which senses when the AC line power fails and connects the emergency lamps to the battery. This practice is quite expensive in that two separate illumination systems must be provided, while only one is used at any given time. Also, such systems provide emergency light, whenever a power failure occurs, whether or not emergency light is needed. Additionally, quite often the emergency lighting system does not provide illumination in the same areas as the regular lighting system. Moreover, the fixtures of such systems are extremely difficult to integrate into an overall decorative lighting scheme.

Such systems using relays or similar electro-mechanical devices to perform automatic switching on a power failure have several disadvantages; relays are subject to mechanical wear after a period of use or likely to be affected adversely by vibration or shock. Relay contacts may fail to make adequate electrical connection after a period of time due to an accumulation of dust or other foreign matter or due to atmospheric corrosion or corrosion caused by sparking. A relay consumes an appreciable amount of power which adds to the running costs of the equipment and which results in a generation of heat that may be undesirable, particularly if the relay is in a compact enclosure. [t is, therefore, advantageous to eliminate the use of relays or other moving parts in an emergency lighting system.

This may be accomplished by the use of static switching circuits which employ semi-conductor devices, such as control rectifiers or the like to perform a switching function. When static switching circuits are used it is common practice to insert the switching element in series with the battery and the load.

The voltage drop introduced by a switching circuit in line with the battery and the lamp and/or inverter is undesirable in that the switching element introduces a voltage drop that subtracts from the voltage delivered by the battery and so reduces the efficiency of the circuit.

For example, a typical silicon control rectifier used as a switching element develops a forward voltage drop of about 1 volt when carrying current in its conducting state. In emergency lighting systems, batteries of relatively low voltages are commonly used for reasons of economy as, for example, 6 volts. Therefore a drop of 1 volt in the switching element represents an appreciable proportion of the supply of the battery voltage and, therefore, the switching element consumes an appreciable proportion of the power delivered by the battery. This consumption of such a substantial proportion of the power delivered by the battery requires that the battery must be of larger capacity and therefore more expensive than would otherwise be necessary to supply the load itself.

Accordingly, the present invention provides a new and improved circuit for emergency lighting systems which overcomes the aforementioned limitations and deficiencies of presently known emergency lighting systems.

Briefly stated, the invention, in one form, comprises an emergency lighting system for electric discharge type lamps in which a battery voltage is inverted to provide alternating AC energy. A static switching element, for example a controlled rectifier (thyristor), or transistor is coupled between the battery and the inverter so as to switch the feedback circuit of the inverter instead of the power line thereto. The invention further provides a self-contained fluorescent lighting fixture, which can provide illumination when the AC line power is available and which continues to provide illumination if the line power supply fails.

The present invention is effective to enable the emergency lighting system regardless of whether the lamp is then in operation or, alternatively, it can be connected so that emergency light is provided under power failure conditions only when the lamp is switched on. Additionally a system embodying the invention is so arranged that the circuit which senses AC line power is also effective to provide a battery charging current. An inverter disabling circuit is also responsive to the charging current to disable the inverter so long as AC line power is sensed.

An object of this invention is to provide a new and improved emergency lighting system.

Another object of this invention is to provide a new and improved emergency lighting system of simplified and economical construction.

Another object of this invention is to provide a new and improved emergency lighting system using a DC AC inverter in which a semi-conductor switching device is incorporated in the feedback circuit of the inverter and is arranged to positively switch between inverter inhibiting and enabling conditions when failure of line power is sensed.

A further object of this invention is to provide a low cost, simplified emergency lighting system which may be housed in the same fixture as the lamp which it serves.

The features of the invention which are believed to be novel are particularly set forth and definitely claimed in the concluding portion of this specification. The invention, however, both as to its organization and operation, together with further objects and advantages thereof may best be appreciated by reference to the following detailed description taken in conjunction with the drawings, in which:

FIG. 1 is a schematic wiring diagram of an emergency lighting system employing a fluorescent light; and

FIG. 2 is a cut-away side elevation of a fluorescent lighting fixture which incorporates the circuit illustrated in FIG. 1.

With particular reference to FIG. 1, B is a battery consisting of one or more cells which may be of the primary type (such as dry cells) or of the secondary type (such as re-chargeable nickel-cadmium). This battery B is connected with polarities as shown in FIG. 1 to input terminals 1 and 2 of an inverter, INV, the basic elements of which are described as follows:

T1 is an inverter transformer, equipped with primary windings N1 and N2, feedback windings N3 and N4, and an output or load winding N5, these windings having polarities indicatcd by start marksQThe core of inverter transformer T1 is of any ferro-magnetic material which displays sufficiently low hysteresis and eddy current losses at the operating frequency of the inverter. Ferrite material has been found satisfactory, for example.

The emitter-collector paths of transistors 01 and Q2 are connected in series with the primary windings, N1 and N2, respectively of inverter transformer T1, across the battery B at terminals 1 and 2. During operation of the inverter, the bases of transistors Q1 and Q2 are alternately driven so that if O1 is turned on while Q2 is cut-ofi, and then after a certain time interval, the base drive is reversed so that 01 becomes cut-off and Q2 is turned on for a certain time interval, and these alternations of base drive are continued, then substantially all the voltage of battery B will alternately be impressed across windings N1 and N2. The polarities of these windings are such that an alternating magnetic flux, having an approximately square wave shape, will be created in the core of inverter transformer T1. By transformer action, alternating voltages will be induced in other windings on the core of transformer T1.

Feedback windings N3 and N4 are joined to a feedback terminal at point 7 and are connected to the bases of transistors Q1 and 02 at points 5 and 6 respectively. When a path for current flow exists between point 7 and either point 1 or point 2, then windings N3 and N4 can provide suitable base drive for transistors Q1 and Q2 and the combination of inverter transformer T1 and transistors Q1 and Q2 forms a self-saturating inverter of a type well known to those skilled in the art.

However, if the feedback terminal point 7 is electrically isolated from point 1 and point 2, then no path exists for the fiow of base current in transistors Q1 and 02 (except for negligible reverse leakage currents through the base-emitter paths) and the inverter can not operate.

The specific inverter circuit herein described is intended as exemplary and not limitative of the invention. Whereas the inverter circuit has been described using transistors of the p-n-p type, other transistors of the n-p-n type or controlled rectifier may equally well be used with appropriate modifications to the circuit. Also, whereas an inverter circuit has been described in which the emitters of the transistors are connected to the primary windings of the inverter transformer and the collectors of the transistors are connected to a common point, a circuit in which the collectors of the transistors are connected to the primary windings of the inverter transformer, and the emitters are connected to a common point may equally well be used. Also, whereas an inverter circuit using two transistors has been described, other circuits in which one transistor and its associated windings are omitted may equally well be used. Additionally, the inverter may be of any suitable free running oscillator having a feedback circuit to sustain oscillations.

A static switching circuit which can provide a path for current flow between point 7 and point 2 comprises a resistor R1, controlled rectifier (often referred to, particularly in European terminology, as a thyristor) CR, resistor R2 and the secondary winding N6 of a transformer T2 having a primary winding N7. For the purposes of this description, resistor R2 is considered as including the resistance of the winding N6. The primary winding of transformer T2, N7, is connected to terminals 8 and 9 which are connected to the AC power supply. A capacitor, C1, is connected between the gate 10 and the cathode ll of controlled rectifier CR. The battery positive terminal l is connected through a resistor, R4, in parallel with a diode, D2, to the gate 10 of controlled rectifier CR. The battery positive terminal 1 is also connected through a resistor, R3, in series with a diode, D1, to the cathode ll of controlled rectifier CR.

The operation of this static switching circuit is as follows: When an alternating voltage exists across the AC power supply line terminals 8 and 9, the primary winding N7 of transformer T2 is energised, and a voltage which is arranged to be commensurate with the voltage of the battery B is developed across the secondary winding N6 of transformer T2.

-When the potential of point 13 is positive with respect to point 2, and is higher than the voltage of the battery B, current flows from winding N6 through resistor R2, diode D1, resistor R3 and the battery B is such a direction as to charge the battery B. The amount of charging current is determined by the magnitude of the voltage developed across winding N6, the values of resistors R2 and R3, and the voltage and internal resistance of battery B. The combination of transformer T2 and diode D1 forms a half-wave rectifier circuit in which current flows during part of each cycle of the AC power supply voltage applied to terminals 8 and 9.

When current is flowing into battery 8, the potential of point 11 is positive with respect to point 1 due to the voltage drop across resistor R3 and the diode D1. Current attempts to flow from point 11 through controlled rectifier CR in the reverse direction (cathode to anode) to point 12, through resistor R1 to point 7, through windings N3 and N4 to points and 6, and through the base-collector paths of transistors Q1 and Q2, to point 2. In actual fact, no current will flow through this path because it is blocked by controlled rectifier CR. If the controlled rectifier CR has previously been in its conducting state, it will be turned off.

Also, when current is flowing into battery B and the potential of point 11 is positive with respect to point 1, a charging current will flow from point 11, through capacitor C1 to point 10, and through diode D2 to point 1. This charges capacitor C1 in such a direction that point 10, which is connected to the gate of controlled rectifier CR, becomes negative with respect to point 11, which is connected to the cathode of the controlled rectifier CR. As long as the charge on capacitor C1 remains with this polarity, the controlled rectifier CR, having been turned off, cannot turn on, because its gate is at a negative potential with respect to its cathode.

The aforementioned conditions apply during the forward half cycles of the AC power supply voltage, when the potential of point 13 is positive with respect to point 1. During the reverse half cycles, the potential of point 13 becomes negative with respect to point 1. Diode D1 blocks the flow of reverse current from battery B into winding N6. Resistors R2 and R4, in combination with capacitor C1 form a timing circuit. Current flows from battery B through point 1 and resistor R4 into capacitor C1, through resistor R2 and winding N6 in such a direction as to diminish the charge previously established on capacitor C 1, and ultimately to charge capacitor C1 in the opposite direction with point 10 becoming positive with respect to point 11. However, by selecting suitable values of resistors R2 and R4, and capacitor C1, the time constant of this timing circuit can be made long in comparison with the period of alternations of the AC power supply voltage. Consequently, the voltage across winding N6 will again reverse so that points 13 and 11 again become positive with respect to point 1 before the charge on capacitor C1 is diminished to zero. The gate of the controlled rectifier CR, point 10, is thus maintained at a negative potential with respect to its cathode, point 11,

throughout the reverse half cycles of the AC power supply voltage, and so controlled rectifier CR does not turn on.

Therefore when the AC power supply voltage is available at points 8 and 9, controlled rectifier CR is maintained in its turned off state and point 7 is effectively open-circuited from both points 1 and 2 with the result that the inverter INV is inoperative.

When the AC power supply applied to terminals 8 and 9 fails, no voltage is induced across winding N6 and the potential of point 13 becomes the same as that of point 2. Current from the battery B flows through point 1, resistor R4, capaci tor C1, resistor R2 and winding N6, in such a direction as to reverse the previously established charge on capacitor C1 and to cause point 10 to become positive with respect to point 11. The gate of controlled rectifier CR, point 10, becomes positive with respect to its cathode, point 1], which causes it to turn on and provide a path for the flow of current from its anode, point 12 and its cathode, point 11. In this way, point 7 is effectively connected through resistor R1, controlled rectifier CR, through resistor R2 and winding N6 to battery negative, point 2, thus causing the inverter lNV to operate and to remain operating until AC power is restored to terminals 8 and 9. The current which flows through the controlled rectifier CR from its anode to cathode is equal to the sum of the base currents of transistors Q1 and Q2 and is, of course, less than the total current delivered by the battery B to the inverter INV.

If it is desired to prevent operation of the inverter when a power failure occurs, at times when lighting is not required, this can be done by connecting a switch in series with the anode circuit of the controlled rectifier CR (for example at point 12). When this switch is open the inverter is inhibited. This switch can conveniently be of the double pole type, having its second pole connected in the AC power supply line to one tenninal of the ballast BAL at point 16 or 17, so that the same switch serves to control the regular lighting when power is available.

A static switching circuit has been described which senses the presence of voltage at the AC power supply terminals, 8 and 9, and when this voltage exists, inhibits operation of the inverter INV. When the AC power supply applied to terminals 8 and 9 fails, the inverter lNV operates. ln this static switching circuit, the switching element CR needs only to be rated to carry the base drive current of transistors 01 and 02, which is approximately equal to the total current delivered by the battery B divided by the current gain of the transistors Q1 and O2. in a typical application, this current has been found to be less than one tenth of the total current delivered by the battery. The power dissipated in the switching element CR is therefore less than that which would be dissipated in a switching element carrying the total current delivered by the battery, and the efi'rciency of the circuit is therefore higher and the rating and cost of both the switching element and the battery can therefore be lower.

When the inverter INV is operating, an alternating voltage is induced in winding N5 of transformer T1. This winding N5, is connected in series with a capacitor, C2, to a fluorescent or similar gas discharge lamp, FL. By suitable design of the inverter transformer T1, the frequency of operation of the inverter is arranged to be high in comparison with the frequency of the AC power supply. In typical applications, inverter operating frequencies between 5,000 Hz. and l5,000 Hz. have been found suitable. The number of turns on winding N5 is arranged so that the output voltage developed across this winding is high in comparison with the voltage needed to maintain the lamp FL alight; a voltage of 500 volts being typical. The value of capacitor C2 is selected so that it presents sufficient reactance to limit the current through the lamp FL to a suitable value.

The lamp FL is also connected to the output terminals, 14 and 15, of a ballast, BAL, which is of the type commonly used for such lamps. In the diagram FIG. 1, a typical schematic circuit of such a ballast is shown, but this is indicative only of one particular type of ballast, and ballasts having different internal connections can be used. In some cases, additional connections may exist between the ballast and the lamp to provide power for heating filaments in the lamp for the purpose of starting and sometimes of maintaining the lamp alight. The input terminals, 16 and 17, of the ballast BAL are connected to a source of AC power suitable for operation of the lamp which source can, as indicated in the diagram FIG. 1, be the same source of power as that applied to terminals 8 and 9, but not necessarily so.

When AC power is available at terminals 8 and 9, and at terminals 16 and 17, the lamp FL is alight drawing power for its operation through the ballast BAL from the AC power supply in the conventional manner. A circuit exists in parallel with the lamp FL comprised of capacitor C2 in series with winding NS. The reactance of capacitor C2 at the frequency of the AC power supply is so high that the current taken through capacitor C2 and winding N5 is negligibly small compared to the current taken by lamp FL, and therefore does not appreciably affect the efficiency of the lamp, nor does it adversely affect the inverter INV.

When the AC power supply applied to terminals 8 and 9, and 16 and 17 fails, the lamp FL remains alight using power supplied by the battery B to the inverter INV which has been turned on by the static switching circuit herein described. A circuit now exists in parallel with the lamp FL comprised of the ballast BAL and, through it, all other loads connected to the AC power supply. Since the operating frequency of the inverter INV is high compared to the frequency of the AC power supply, the ballast presents a high impedance to the high frequency voltage appearing across the lamp FL and therefore draws a relatively small current from the inverter INV. Satisfactory operation is thus obtained.

The specific static switching circuit which has been described herein is intended as exemplary and not limitative of the invention. Whereas the description and FIG. 1 of the diagrams refer to a switching element in the form of a controlled rectifier having gate, cathode and anode electrodes connected to points 10, 11, and 12 respectively, other types of semi-conductor could equally well be used such as for example, a n-p-n transistor with base, collector and emitter connected to points l0, l1 and 12 respectively. Whereas transformer T2 is described and shown in FIG. 1 as having two electrically separate windings, an auto-transformer having one tapped winding could equally well be used. Also, whereas the description and FIG. 1 refer to half-wave rectification using one diode, D1, other circuits using more than one diode and providing full-wave rectification could equally well be used.

In one embodiment of the invention, the components herein described can be arranged in a self-contained lighting fixture as typically illustrated in the diagram FIG. 2. The inverter transistors, Q1 and 02, are mounted on a heat sink 18 which is fixed to a base 19. The static switching circuit components R1, R2, R3, R4, C1, CR, D1, D2 and capacitor C2 are mounted on a component board 20 which is in turn fixed to the heat sink 18. The remaining components, battery B, ballast BAL, inverter transformer T1, transformer T2 and also two lampholders 21 and 22 are fixed to the base 19. A cover 23, which may also serve the purpose of a reflector, completes the enclosure.

Whereas a fixture has been described and illustrated in FIG. 2 with reference to one lamp, and one ballast, other types of fixture which incorporate more than one lamp, possibly more than one ballast, and possibly additional functional or decorative parts such as diffusers, reflectors and so on could equally well be used.

It may thus be seen that the objects of the invention set forth above as well as those made apparent from the foregoing description are efiiciently attained. Other embodiments of the invention and modifications to the disclosed embodiment thereof which do not depart from the spirit and scope of the invention may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all modifications and embodiments of the invention which do not depart from the spirit and scope thereof.

Iclaim:

I. An emergency lighting system for use with an electric discharge lamp including a ballast and having terminals adapted to be connected across an AC line, comprising:

a free running oscillator having a feedback circuit for sustaining oscillations,

a semi-conductor switching device in said feedback circuit for enabling or inhibiting operation of said oscillator,

a battery for supplying unidirectional energy to said oscillator,

a first circuit for sensing AC line power across said terminals and for charging said battery,

a second circuit connected to said first circuit including a capacitor adapted to be charged from said first circuit and to back bias said switching device and prevent conduction thereof and thereby inhibit operation of said oscillator,

said capacitor being adapted to discharge upon failure of the AC power and being ineffective to back bias said switching device, whereby said semi-conductor device is rendered conductive and operation of said oscillator is enabled, and circuit means for connecting the output of said oscillator across the lamp and ballast.

2. The system of claim 1, wherein the time constant of said second circuit is selected to be substantially longer than a cycle of the AC power so that the charge on said capacitor is maintained at a sufficient value to back bias said semi-conductor device when AC line power is present.

3. The system of claim 1, wherein said emergency lighting system, said lamp and said ballast have a common housing.

4. The system of claim 1, wherein said sensing means includes a transformer connected across the AC line, a unidirectional conducting device in circuit with the secondary of said transformer and connected across said battery to provide a charging current thereto from the AC line.

5. The system of claim 1, further including circuit means connecting said battery to said semi-conductor device to forward bias said device upon discharge of said capacitor.

6. The system of claim 1, wherein said oscillator is of the type having a saturating core, a load winding and feedback windings on said core and a pair of semi-conductor switching devices providing circuit paths to said load winding and switched ON and OFF by the voltage developed across said feedback windings upon saturation of said core, and said semiconductor device is in circuit with said feedback windings.

7. The system of claim 1, wherein said oscillator includes a transformer in its output circuit, the secondary of said transformer connected in series with a capacitor across the lamp.

8. The system of claim 1, wherein said semi-conductor device is a controlled rectifier having a gate electrode, said gate electrode being connected to the positive terminal of said battery to forward bias said device upon discharge of said capacitor.

9. A power-pack assembly connectable to an electric discharge lighting fixture having an enclosure provided with means for mounting an electric discharge lamp and containing a ballast and electrical connections for the lamp to function from an AC power supply, said assembly including AC line power sensing means, a battery, a charger for the battery, an inverter, and a switching circuit, all mounted within a casing of a size and shape for insertion into the enclosure of the lighting fixture, said assembly having connecting means for connecting it to the lamp, to the ballast and to the AC power supply, whereby the assembly supplements the fixture so that the lamp provides regular light when the AC power supply is available and continues fed by the battery automatically to provide emergency light when the AC power supply fails, said inverter comprising a free-running oscillator having a feedback circuit for sustaining oscillations, said switching circuit being connected in said feedback circuit and to sense the available AC power, said switching circuit being effective to open said feedback circuit and render said oscillator inoperative when the availability of AC power is sensed from the supply. 

1. An emergency lighting system for use with an electric discharge lamp including a ballast and having terminals adapted to be connected across an AC line, comprising: a free running oscillator having a feedback circuit for sustaining oscillations, a semi-conductor switching device in said feedback circuit for enabling or inhibiting operation of said oscillator, a battery for supplying unidirectional energy to said oscillator, a first circuit for sensing AC line power across said terminals and for charging said battery, a second circuit connected to said first circuit including a capacitor adapted to be charged from said first circuit and to back bias said switching device and prevent conduction thereof and thereby inhibit operation of said oscillator, said capacitor being adapted to discharge upon failure of the AC power and being ineffective to back bias said switching device, whereby said semi-conductor device is rendered conductive and operation of said oscillator is enabled, and circuit means for connecting the output of said oscillator across the lamp and ballast.
 2. The system of claim 1, wherein the time constant of said second circuit is selected to be substantially longer than a cycle of the AC power so that the charge on said capacitor is maintained at a sufficient value to back bias said semi-conductor device when AC line power is present.
 3. The system of claim 1, wherein said emergency lighting system, said lamp and said ballast have a common housing.
 4. The system of claim 1, wherein said sensing means includes a transformer connected across the AC line, a unidirectional conducting device in circuit with the secondary of said transformer and connected across said battery to provide a charging current thereto from the AC line.
 5. The system of claim 1, further including circuit means connecting said battery to said semi-conductor device to forward bias said device upon discharge of said capacitor.
 6. The system of claim 1, wherein said oscillator is of the type having a saturating core, a load winding and feedback windings on said core and a pair of semi-conductor switching devices providing circuit paths to said load winding and switched ON and OFF by the voltage developed across said feedback windings upon saturation of said core, and said semi-conductor device is in circuit with said feedback windings.
 7. The system of claim 1, wherein said oscillator includes a transformer in its output circuit, the secondary of said transformer connected in series with a capacitor across the lamp.
 8. The system of claim 1, wherein said semi-conductor device is a controlled rectifier having a gate electrode, said gate electrode being connected to the positive terminal of said battery to forward bias said device upon discharge of said capacitor.
 9. A power-pack assembly connectable to an electric discharge lighting fixture having an enclosure provided with means for mounting an electric discharge lamp and containing a ballast and electrical connections for the lamp to function from an AC power supply, said assembly including AC line power sensing means, a battery, a charger for the battery, an inverter, and a switching circuit, all mounted within a casing of a size and shape for insertion into the enclosure of the lighting fixture, said assembly having connecting means for connecting it to the lamp, to the ballast and to the AC power supply, whereby the assembly supplements the fixture so that the lamp provides regular light when the AC power supply is available and continues fed by the battery automatically to provide emergency light when the AC power supply fails, said inverter comprising a free-running oscillator having a feedback circuit for sustaining oscillations, said switching circuit beIng connected in said feedback circuit and to sense the available AC power, said switching circuit being effective to open said feedback circuit and render said oscillator inoperative when the availability of AC power is sensed from the supply. 